The Effect Of Placental Trophoblasts On Tolerogenic Immune Mechanism Through Regulating Th1,Th2 And Th17 Immunity

Maternal tolerance is necessary for the mother to tolerance the semi-allogenic conceptus containing paternal heritages and not to provoke maternal immunity against the fetal-placental unit for as long as 40 weeks.A lot of hypotheses to explain fetal tolerance have been developed and investigated for 50 years.The mechanisms of fetal tolerance are plural and working at different periods of time in embryonic and fetal development.These tolerance mechanisms act at distance on the maternal immune system or locally at the placental level.Maternal tolerance to the fetus is established as the occurrence of placental trophoblasts and is abrogated as the disappearance of trophoblasts,suggested that trophoblast is the initiator of the maternal immune tolerance. The basis of maternal immune tolerance is one kind of peripheral tolerance and the tolerance of T cells at the fetal-maternal interface is the basis of maternal immune tolerance.Nowadays,the expression of HLA-G and FasL by the trophoblasts and the shift of cytokine balance to Th2 are some of the mechanisms.However,the underlying mechanism was very much complicated and is far from being understood.Pregnancy is a state of Th2 phenomenon."Th2-phenomenon",Th1/Th2 balance with a shift to predominant Th2-type immunity,is in favor of Th2 cells which produce IL-4,IL-5,IL-6,IL-9,IL-10,TGF-αand reduce the production of pro-inflammatory molecules,like IL-2 and IFN-γ,which can endanger fetus survival.These results observed in pregnant mice demonstrated Th1/Th2 immunity alterations with a shift to predominant Th2-type immunity.On the other hand,predominant Th1-type immunity in pregnant mice is correlated with spontaneous abortions.Similar observations were then obtained in human.It was shown that spontaneous abortions in pregnant women might be associated with Th1-type immunity to trophoblast.Although much effort has been made in searching for pregnancy that is a Th2-phenomenon,however,the mechanisms involved in the establishment of Th2 dominance at the maternal-fetal interface remains largely unknown.We previously reported that placental trophoblast inhibited T lymphocyte,partly by the degradation of tryptophan via IDO expressed by trophoblast,and identified 548 proteins produced by trophoblats with proteomic approaches.Among the identified substances were proteins that inhibit Th1 immunity and/or enhance Th2 immunity,at least including glycodelin(also named placental protein 14,PP14),thrombospondin-1 (TSP-1),galectin-1,pigment epithelium-derived factor(PEDF),transforming growth factor-beta(TGF-β).In addition,estradiol and progesterone that were also produced by trophoblast inhibit Th1 immunity.Our findings indicate that trophoblast product might be a mixture of substances with immunoregulatory properties,that the inhibitory effect on T lymphocyte by trophoblast might be the result of synergistic action of these immunoregulatory substances and that inhibition of Th1 immunity and enhancement of Th2 immunity might be among the mechanisms involved in establishing Th2 dominance at the matemal-fetal interface.Mechanisms involved in the matemal tolerance to fetus are complex.It has been reported that normal human pregnancy is associated with Th17 immunity. IL-17-producing Th17 cells,defined a previously unknown CD4+ effector T-cell lineage,which develop via cytokine signals distinct from the Th1/Th2 and Treg lineages. Emerging evidence indicates that Th17 cells probably play a central role in inflammation,autoimmunity and allergy.Most knowledge about Th17 cells derives from routine studies,while human IL-17 responses have just been recently recognized. However,the involvement of Th17 in the maternal tolerance to fetus has never been investigated.Objectives:To clarify the effect of placental trophoblasts on T lymphocyte by observing the alteration in the production of cytokines and the expression of specific transcription factors for Th1,Th2 and Th17 immunity in T lymphocyte.Methods:Placental trophoblasts were isolated from chorionic villi of normal pregnancy and conditioned medium was made after 72 hours culture of trophoblast. PBMC were isolated from healthy female donors and T lymphocytes were purified and cultured in the presence or absence of conditioned medium.Enzyme-linked sorbent immune assay(ELISA) was used to detect the concentration of IL-2,TNF-α,IFN-γ, IL-4,IL-10 and IL-17 in supernatants of T cell culture and Real-time PCR was used to detect the expression of specific transcription factors for Th1 immunity(T-bet and STAT-4),Th2(GATA-3 and STAT-6) and Th17(RORC) in T lymphocyte.Results:The level of IL-2,IFN-7,TNF-αand IL-17 was significantly decreased (P=0.009,0.02,0.003 and 0.0001,respectively) when the T lymphocytes were cultured in conditioned medium compared with control medium,while the level of IL-10 and IL-4 were comparable.The presence of conditioned medium decreased the ratio of Th1/Th2 as indexed by IL-2/IL-4,IFN-γ/IL-4,TNF-α/IL-4,IL-2/IL-10,IFN-γ/IL-10 and TNF-α/IL-10(P=0.002,0.04,0.007,0.03,0.047 and 0.002,respectively) compared with control medium.The expression of GATA-3 and STAT-6 were significantly increased(P=0.0028 and 0.02,respectively) and STAT-4 was reduced(P=0.006) when T cells were cultured in conditioned medium,while the expression of T-bet and RORC were comparable.Conclusion:Placental trophoblast-induced shift of Th1/Th2 balance toward Th2 and inhibition of Th17 might be among the mechanisms involved in maternal tolerance to fetus.